Individual iron club heads in a set typically increase progressively in face surface area and weight as the clubs progress from the long irons to the short irons and wedges. Therefore, the club heads of the long irons have a smaller face surface area than the short irons and are typically more difficult for the average golfer to hit consistently well. For conventional club heads, this arises at least in part due to the smaller sweet spot of the corresponding smaller face surface area.
To help the average golfer consistently hit the sweet spot of a club head, many golf clubs are available with cavity-back constructions for increased perimeter weighting. Perimeter weighting also provides the club head with higher rotational moment of inertia about its center of gravity. Club heads with higher moments of inertia have a lower tendency to rotate caused by off-center hits. Another recent trend has been to increase the overall size of the club heads, especially in the long irons. Each of these features increases the size of the sweet spot, and therefore makes it more likely that a shot hit slightly off-center still makes contact with the sweet spot and flies farther and straighter. One challenge for the golf club designer when maximizing the size of the club head is to maintain a desirable and effective overall weight of the golf club. For example, if the club head of a three-iron is increased in size and weight, the club may become more difficult for the average golfer to swing properly.
In general, the center of gravity of the cavity-back clubs is moved toward the bottom and back of the club head. This permits an average golfer to get the ball up in the air faster and hit the ball farther. In addition, the moment of inertia of the club head is increased to minimize the distance and accuracy penalties associated with off-center hits. In order to move the weight down and back without increasing the overall weight of the club head, material or mass is taken from one area of the club head and moved to another. One solution has been to take material from the face of the club, creating a thin club face. Examples of this type of arrangement can be found in U.S. Pat. Nos. 4,928,972, 5,967,903 and 6,045,456.
However, professional tour players and low handicap players, who can consistently hit the balls on the club's sweet spot, prefer muscle-back type clubs for the visual effect of a smaller head and better workability. Workability is a function of the size of the club head, the center gravity being closer to the hosel axis, the thinner sole and the reduced offset between the hosel and the hitting face. Workability is the ability to shape the shots and to control the trajectory's height.
Muscle-back clubs generally have lower inertia and higher center of gravity than cavity-back clubs. Muscle-back clubs, such as Kenneth Smith's Royal Signet clubs and Mizuno's MP-33 irons concentrate the club's weight near the sweet spot, thereby reducing its inertia. Also since the club's weight is not moved to the perimeter or to the sole, the conventional muscle-back club does not have as large a sweet spot or low center of gravity as the cavity-back club. Some of the commercially available muscle-back clubs are using multiple materials to change the mass properties. For example, the Bridgestone EC603 Pro iron clubs have a stainless steel body with a heavy tungsten insert in the lower portion of the back of the club (i.e., in the muscle portion of the club), and a urethane insert for vibration damping. Similarly, the Bridgestone Tanbec TB-2 has a titanium body and a heavy beryllium copper insert in the lower portion of the back of the club. However, these heavy inserts reduce the inertia of the club.
Hence, there remains a need for muscle-back clubs that have improved mass properties, such as higher inertia and better location of the center of gravity.